Radiography has been used for over ninety years for capturing medical images (see L. K. Wagner et al. in "Imaging Processes and Materials" J Sturge et al (Ed.), Van Nostrand Reinhold, New York, 1989). In conventional radiography, the x-ray image is captured by a flat sheet of light-emitting phosphor material which is commonly called a screen. The screen emits light when stimulated by x-rays. The light emitted by the screen exposes a silver halide film that stores the image. Typically, the silver halide film is sandwiched between two phosphor screens.
In spite of the successful applications of the conventional film-screen system for radiography, there are many deficiencies (see L. K. Wagner et al. supra). For example, the exposure range of the film-screen system is limited and this sometimes results in over- or underexposure of the film. The exposure display latitude and the contrast of the display of the film-screen system are also limited. In addition, the required chemical processing of the film is inconvenient.
Digital radiography provides an alternative solution to the problems of conventional radiography. Digital image receptors are often capable of capturing a wide range of image information. The useful exposure display latitude is often superior to film, because the variable window level and contrast display of digital images eliminate the display limitations of the film. The ability to improve image by software manipulation gives further flexibility to the digital techniques. However, digital radiography often compromise on spatial resolution.
One form of digital radiography uses a photo-stimulable phosphor (PSP). The PSP is exposed to x-rays to produce a latent image in the form of a varying density distribution of trapped electrons. The plate is then scanned by a laser of relatively long wavelength, usually in the red region of the visible spectrum. The electrons are driven out of their trapped levels by the laser and return to the ground state by emitting a high energy photons (typically green). The high energy photons are then detected imagewise by a photodetector. The main disadvantage of the PSP technology is the slow speed limited by scanning a laser from pixel to pixel. The spatial resolution is also limited by light scattering from the phosphor screen which consists of phosphor powders.
Another form of digital radiography uses x-ray sensitive photoconductors. X-ray photoconductor is deposited as a thin film on a conducting substrate such as aluminum or indium tin oxide glass (ITO). The film is first charged positively or negatively. Exposure to x-ray photons discharges the film imagewise. The residual charges on the film can then be read out imagewise by a detector. Several read-out methods have been developed for this purpose. The photo-induced discharge method uses a scanning laser to discharge the voltage imagewise, and the discharge is detected by an electrometer (see J. A. Rowlands et al., Med. Phys., 18, 421 (1991)). Another method uses a scanning electrometer array to directly measure the voltage imagewise (see W. Hillen et al. in Medical Imaging II, R. H. Schneider et al. (Ed.), SPIE 914, 253 (1988)). The residual voltage can also be read out directly imagewise by a large-area thin-film transistor array (see W. Zhao et al., Proceedings of the Electrochemical Society Meeting, Vol. 92-2, p. 791, Toronto, Canada, Oct. 11-16, 1992).
The principle of x-ray photoconductivity is based on the ability of high energy x-ray radiation to ionize matter. When an atom absorbs an x-ray photon, it is ionized and ejects high-energy electrons. The high-energy electrons travel through the material and cause more ionization until they are thermalized. The overall result of the absorption of x-ray photons by matter is the formation of tracks of ionized species. The distribution of ionized species can be very inhomogeneous, often concentrated in regions called spurs, blobs, or tracks. The exact distribution depends on the energy of the radiation and the material. (A detailed discussion can be found in J. W. T. Spinks et al., "An Introduction to Radiation Chemistry" Wiley, New York, 1976.)
X-ray photoconductors can be used as imaging elements in radiography. The x-ray photoconductive film is first charged and then exposed to x-ray radiation imagewise. X-ray photons generate ionized species (charges) as described above. If the material is capable of transporting charges, the absorption of x-ray causes discharge imagewise. The residual surface charges can then be read imagewise by various techniques such as laser scanning, electrometer array, and thin film transistor array.
A useful x-ray photoconductive material therefore has to have the following properties. First, it has to be a good insulator in the dark (i.e., low dark conductivity), capable of forming large area thin film and sustaining high electric field. Most of the inorganics have problems fulfilling these requirements. Good x-ray absorbing inorganics such as HgI.sub.2 and BiI.sub.3 usually have small band-gaps which means high dark conductivity due to thermal excitation of carriers. Large area thin-films of good x-ray absorbing inorganics are difficult to fabricate and they usually cannot sustain large electric field due to high dark conductivity and the presence of defects. On the other hand, organic polymers excel in this area. They have superior dielectric strength and can be fabricated into large area thin-films by well-established methods such as spin-coating or thermal pressing.
The next important requirement for a good x-ray photoconductor is large x-ray absorption cross section. This is a requirement which organics fail totally but inorganics excel. No organic polymer has an x-ray absorption efficiency good enough for practical applications. On the other hand, the x-ray absorption efficiency of inorganics can be very large, and increases approximately with increasing atomic number. The x-ray absorption cross sections of various elements at different x-ray energies have been tabulated (see CRC Handbook of Chemistry and Physics, 74th edition, D. R. Lide, ed., CRC Press, Boca Raton, Fla., 1993-94, pp. 10-287-10-289).
Finally, a good photoconductor requires that the generated carriers move through the film without significant trapping. This property is unpredictable for either organics or inorganics and depends on the material, the preparation procedures, the presence of impurities, defects.
In sum, a useful x-ray photoconductive film should be a good insulator in the dark and capable of sustaining high electric field, should have high x-ray absorption cross section, and should permit generated carriers to move through the film without being significantly trapped. In addition, if photo-induced discharge is employed as the read-out method, the material should be a good uv-vis-IR photoconductor.
Good x-ray photoconductors are difficult to prepare. Inorganics containing heavy elements absorb x-ray photons efficiently, but are difficult to fabricate into large area, good quality thin-films. Furthermore, they usually have high dark conductivity and cannot sustain high electric fields. Polymers can be fabricated into good quality thin-films, have low dark conductivity and high dielectric strength, but are inefficient x-ray absorbers.
At present selenium is the only useful x-ray photoconductive material that can meet these stringent requirements. It also has many drawbacks. The x-ray absorption efficiency of selenium is not very high. Good quality selenium thin-films without carrier trapping sites are notoriously difficult to prepare. The toxicity of selenium and issues regarding its safe handling are of great concern. There is continuing interest in developing other useful materials having x-ray photoconductivity.
U.S. Pat. No. 4,738,798, discloses compositions of semiconductor clusters doped into ethylenemethacrylic acid co-polymer. U.S. Pat. No. 5,238,607, disclosed photoconductive polymer compositions containing semiconductor nanoclusters selected from the group consisting of IIB-VIB, IIB-VB, IIIB-VB, IIIB-VIB, IB-VIB, and IVB-VIIB semiconductors.